The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created within the plasma impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate on which they are being deposited at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices with a high depth to width aspect ratio can bridge over the opening causing undesirable cavities in the deposition layer. To prevent such overhang, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of overhangs. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease unwanted overhang formation in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil induces electromagnetic fields and generates a high density plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
In many high density plasma applications, it is preferable for the chamber to be operated at a relatively high pressure so that the frequency of collisions between the plasma ions or plasma precursor gas atoms and the deposition material atoms is increased to increase thereby the resident time of the sputtered material in the high density plasma zone. As a consequence, the likelihood that deposition material atoms may be ionized is increased thereby increasing the overall ionization rate. However, scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms often causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions.
In order to improve the uniformity of deposition, the coil which is used to couple RF energy into the plasma has been adapted to sputter material from the coil onto the workpiece to supplement the material being sputtered from a target onto the workpiece. The coil may be positioned adjacent to the substrate so that material sputtered from the coil is deposited primarily onto the periphery of the workpiece. One end of the coil is coupled to an RF generator and the other end of the coil is coupled to the system ground, typically through a blocking capacitor to develop a DC bias on the coil to facilitate sputtering of the coil. If the coil is a single turn coil, the ends of the coil are typically positioned close together but spaced by a gap (typically on the order of xc2xc inch (4-8 mm)) to prevent a short between the RF generator and the blocking capacitor which would bypass the coil.
Although sputtering material from the coil onto the workpiece can improve the uniformity of deposition, it has been noted by the present applicants that nonuniformities in the deposition can nonetheless occur. Accordingly, further improvements in deposition uniformity is desired.
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter depositing a layer which obviate, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, a plasma generating apparatus which inductively couples electromagnetic energy and sputters material from a coil which has two spaced but overlapping ends, and a pair of RF feedthroughs connected to the coil ends, also positioned in an overlapping fashion. As a result, the current path around the coil from one feedthrough to the other feedthrough need not have a circumferential or azimuthal gap in the vicinity of the coil ends. It has been recognized by the present applicant that the gap which normally spaces the two RF feedthroughs of a prior single turn coil may cause a nonuniformity in the plasma density which may adversely affect the uniformity of deposition onto the substrate closest to the coil gap as compared to other portions of the substrate. By reducing or eliminating the circumferential gap in the current path at the ends of the coil, it is believed that the coil can provide a more uniform plasma density around the circumference of the coil, even adjacent to the ends.
In several illustrated embodiments, the coil ends and associated RF feedthroughs circumferentially overlap in a direction generally parallel to the axis of the substrate holder and the substrate supported on the holder. In alternative illustrated embodiments, the coil ends can circumferentially overlap in a radial direction. In each of the embodiments described herein below, it is believed that such overlapping of the RF feedthrough positions adjacent to the coil ends can improve the quality of the layer deposited onto the substrate.